Although Amazon.com hasn’t disclosed the exact means of determining the Best Sellers ranks, as of 2015, it uses a combination of recent and historical sales revenue data, according to ExportX. For each item, Amazon.com chooses its most popular subcategories and ranks it relative to other items in those subcategories.
Amazon.com updates its Best Sellers list on an hourly basis, and it places more weight on the more recent sales, meaning that items that boast high sales in a recent period can overtake other items even if the latter sold more overall. Although Amazon.com takes into consideration the number of units sold, it prioritizes sales revenue over it. The Amazon Best Sellers rank doesn't reflect how well an item sells relative to other similar items that aren't part of the same subcategories that are displayed in the Product Details section on its product page.
Despite the hourly updates, there is typically a delay of 2 to 3 hours between a sale and its impact on the Best Seller rank. Furthermore, the ranking of certain items, such as books, that are not in the top 10,000 gets updated only once per day. Amazon.com also uses historical sales data in its predictive algorithms do determine the current rank, allowing low ranked items to jump high on the list after a relatively minor boost in sales.